Customizing Feed in child theme – syndication help

first things first, i have created a working child theme, in the folder i have a .css & funtion.php file. do i need to create feed.php … im new to this but have been able to do things with good forum support. i need step by step instruction becuase i have just started to work with code

i want to control # of post shown from different categories ( blue category shows all 10 post but want green category to show 1 – the most recent, have been trying for days but dont know what im doing..

this is what i was try, placing code in my function.php ( fail )

remove_all_actions( 'do_feed_rss2' );
add_action( 'do_feed_rss2', 'green_feed_rss2', 10, 1 );

function green_feed_rss2( $for_post ) {
    $rss_template = get_template_directory() . '/feeds/feed-green-rss2.php';
    if( get_query_var( 'post_type' ) == 'green' and file_exists( $rss_template ) )
        load_template( $rss_template );
    else
        do_feed_rss2( $for_post ); // Call default function
}

/*
Child Theme: twenty child 1
Theme URI: https://www.psychic3000.com/
Description: Child theme for the Twenty Eleven theme
Author: Creed masters
Author URI: https://www.psychic3000.com/
Template: twentyeleven
Version: 3.4.2
*/
my website > https://www.psychic3000.com
My blog > https://www.psychic3000.com/wordpress

im lost & needs lots of help !!!!
any help would be great ..